1. Field of the Invention
The present invention relates generally to Dry Offset printing. More particularly, the invention relates to an enhanced halftone screening method for the Dry Offset printing process.
2. Discussion of the Related Art
The Dry Offset Printing process (sometimes referred to as “Letterset” and “Indirect Relief Printing”) is an offset printing process combining the characteristics of letterpress and offset. Dry Offset printing is similar to offset lithography in that a rubber blanket is used to transfer the image from the printing plate to the container surface. As in the letterpress, the plate used has the image area raised above the surface of the plate Ink is distributed through a series of rollers onto the raised surface of the plate.
The plate transfers the image to the rubber blanket, which prints the entire multicolored copy taken from one or more (i.e., up to ten) color plate cylinders on the container in a single operation. The “Dry” denotation of this offset system serves to differentiate it from the offset system, which uses the incompatibility of water inks to “dampen” the surface of the plate or substrate to prevent ink transfer.
Dry Offset Printing provides the most efficient method for high speed, large volume printing of multi-colored line copy, half-tones and full process art on round, or cylindrical, three dimensional products—such as preformed plastic parts. Dry Offset printing is used primarily to print on products such as tapered cups, tubs and pails, beverage cans, as well as tubes, jars and their respective closures (i.e., caps and lids).
The colors used in offset printing are usually Cyan, Magenta, Yellow and Black (conventionally denoted as “CMYK”). Different percentages of each of these four colors create virtually every color used in offset printing. There are color matching systems, such as the Pantone® system, that allow print buyers to see the color. The code for that color can be entered into the offset printer's computer and it will calculate the percentage of each color to use. Although the specification makes specific reference to the Pantone color matching system, it will be apparent to those skilled in the art of Printing that the methods of the present invention could be employed with other color space system as well.
During the process, ink from an ink fountain is transferred, via a set of rollers, to a relief printing plate having the image area raised above the surface of the plate. Each color used in the job has a separate printing plate. The ink from each printing plate is transferred to a common rubber blanket. Each printing plate color is transferred on top of the preceding color. For example, in the case of a 4-color print job (e.g., Yellow, Red, Blue and Black), the Yellow ink transfers from the plate to the rubber blanket first. Then, the blanket moves forward to receive the ink from the Red plate, and so on. Once the final color, Black, is transferred on top of the other colors on the blanket, the blanket transfers all of the colors simultaneously to the print surface of the round container, where the ink is then cured or dried by either ultra violet lights or an oven.
All colors are transferred in a wet state on top of each other on the common blanket. This causes two major problems: (1) a loss of print contrast; and (2) ink contamination. The loss of print contrast causes the image to look flat, muddy, and lacking brilliant bright color. The ink contamination is caused by the relief plate picking up some of the ink from the blanket of the previous color. This causes the color contaminate to migrate back through the ink rollers to the ink fountain, changing the hue and contaminating the ink purity over the course of the run length.
Current screening technology utilizes four common printing angles. These angles are generally: 45°, 75°, 90° (or 0°), and 105° (or 15°); or, in the case of Flexographic printing, these same angles offset by 7.5°. Each color is assigned a different angle, so that when all four colors are transferred on to the product there will be no offensive pattern such as a moiré (see FIG. 2). However, there is still a less offensive pattern that occurs, commonly referred to as a rosette pattern (see FIG. 3). When each color is screened with one of the four angles, the rosette displays a circle with different color dots overlapping one another. This problem is compounded further when more than four colors are needed. The overlap of the colored dots is the cause of ink contamination. The rosette pattern becomes much more visible the coarser the line screen ruling. An 85-line screen (i.e., 85 dots per square inch)—which is not uncommon in Dry Offset printing—will display a considerable rosette pattern and a substantial loss of fine image detail. Frequency Modulated (FM) Stochastic Screening, which is a random pattern of variable size and irregular shaped dots, has also been tried—to no avail. More detail has been seen with less moirés and less rosette patterns; however, ink contamination and print contrast have remained very poor. The finer stochastic dots also make it virtually impossible to control print consistency over the course of the run length.
Current known technology involved in screening for Dry Offset Printing falls far short of what is desired in the printing community. U.S. Pat. No. 5,010,814 to Shishikura discloses a method of non-overlap, halftone-dot printing of a colored original on the outer surface of a cylindrical container via dry off-set printing. A colored original is color-separated into three primary colors (i.e., Cyan, Magenta and Yellow), and change-to-halftone is performed to give an effect of contact screening with the same angle for the individual separated primary colors. One pixel of print is expressed by three colors at most, i.e., one of the three primary colors, one of three secondary colors, i.e., Cyan-Magenta, Magenta-Yellow and Yellow-Cyan blend colors, and Black. This technology does not allow for maximization of ink on the blanket. Furthermore, it utilizes many extremely small circular slices abutting each other with no gap in between, causing press gain issues, ink contamination and registration problems, resulting in an undesirable grainy look. Additionally, the technology is limited to seven colors (i.e., geared to C, M, Y, CM, MY, YC, and K) and the way that the color Black is created results in a very limited range and detail vis-à-vis the present invention. U.S. Pat. No. 4,998,962 to Edwards discloses a method of printing with at least two printing plates, or their equivalent, wherein associated halftone dots belong to two distinct pluralities, each of which is printed with one of the two printing plates, and each of which are interleaved with the other and printed in a color different from the other, separated by a gap when printed in perfect registration. This technology creates a gap between the overlapping dots, does not replace the color lost from the clipping off of the overlap, does not create a continuous tone look, does not eliminate the rosette pattern, and is extremely limited in the number of colors that can be used.
Yet another known technology is a Dry Offset printing process using a printing apparatus including at least one inker block, each block inking a relief plate corresponding to one color of the final motif to be reproduced on an object, including an inker and a succession of transfer rollers, and a central blanket which can be placed into contact with the object to be decorated. An ink of suitable tackiness is selected, and the temperature of the ink is regulated in the inker to a predetermined temperature between 15° C. and 30° C.
Another existing technology comprises a Dry Offset printing apparatus for high speed printing on cylindrical objects fed either horizontally or vertically to the printer, including a three-roller system having a single roll ink-metering system, plate cylinder, and blanket or print roll. The ink roll is provided with a relatively soft, smooth elastomeric surface, as is the offset print roll blanket. A doctor blade is configured and oriented with respect to the ink roll to meter a uniformly even, thin film of ink onto the roll. Means are provided for evenly distributing the ink film on the ink roll in both horizontal and vertical embodiments of the printer. Means are also provided to prevent excessive ink buildup on print roll when skips occur at the printing station. Rotary transfer means are provided for positively feeding the cylindrical objects to the printer at high speeds in either horizontal or vertical orientation.
The above cited technologies do not solve the problems posed and explained above; therefore, a reliable and repeatable screening method for the Dry Offset printing process capable of addressing the above mentioned problems is still desired.